1. Field of the Invention
The invention relates to a method of transmitting information. Prior to transmission, m-bit information words are converted into n-bit code words. After transmission said n-bits code words are re-converted into m-bit information words. In the words, m&lt;n and m and n are positive integers. In order to obtain a d.c. free transmission the possible code words have been selected in such a way that the running sum ##EQU2## remains restricted over the bits of the generated code words, where x.sub.j =.+-.1 is the value of the j.sup.th bit, regardless of the sequence of information words.
The invention also relates to an encoding device for use in the method.
Moreover, the invention relates to a decoding device for use in the method.
2. Prior Art
Such methods and devices are employed in order to obtain an optimal signal spectrum for transmission. For example, in the case of transmission via a magnetic tape, and also via other media such as cables, the ether, or optical record carriers, it is desirable to have a d.c. free signal and a spectrum with a minimal low-frequency content in order to make room for various control signals such as tracking signals in recording. It is known, inter alia from GB-PS No. 1,540,617 that a d.c. free code is obtained if the running sum over consecutively generated bits remains restricted. This happens when logic "1" and logic "0" are assigned opposite values). Restricted means that there will be no series of input words which gives rise to a series of code word whose running sum increases towards infinity. The simplest method of guaranteeing this is to use as code words only those words which have a disparity equal to zero. Disparity is the sum over the bit value of the code word, i.e. the variation of the running sum caused by this code word. To use code words having zero disparity means to use only code words comprising as many ones as zeros. An example of this is the "biphase" code in accordance with the table in FIG. 2. Another simple method, which results in a larger number of possible code words and which is therefore more efficient, is described in said British Patent Specification. In addition to code words of zero disparity, code words of a specific non-zero disparity are permissible. Both a code word of positive disparity and a code word of opposite disparity (for example +2 and -2) may be assigned to the relevant input words. One of these two code words is chosen according to the digital sum value (running sum) over the preceding code words, i.e. so that the absolute value of this digital sum value remains restricted. The simplest way to do this has been to select a first set of code words and to generate the associated set of code words by inverting the words of the first set.